In recent years, as demand of mobile communications represented by a portable telephone is expanded, a communication system having high efficiency in use of frequency is needed.
A Code Division Multiple Access (CDMA) system is one of the communication systems which meet the demand. In such a CDMA system, fine control of the transmitting electric power in a mobile station is required. Thus, it is necessary that a variable gain amplifier which is used in an IF (intermediate frequency) section operates to control a wide range gain of 70dB or more. In order to perform such wide range gain control, exponential change of the gain control signal level is necessary. Moreover, the gain control characteristic is desired to be stable independent on temperature. Also, it is required that a portable telephone should be small and lightweight. Integrated circuits are used in portable telephones to meet such requirements.
As shown in the U.S. Pat. Publication No. US2002/0070788 A1 (corresponding to Japanese Patent Publication (Kokai) P2002-92541), this and other inventors proposed an exponential conversion circuit using MOS transistors of a weak inversion area previously. The converting characteristic of the exponential conversion circuit is not influenced by temperature change. A CMOS type variable gain circuit using such an exponential conversion circuit was also proposed in the US Patent Publication.
The exponential conversion circuit shown in the US Patent Publication contains first and second voltage conversion circuits with controllable gains, a gain control voltage generating circuit and an exponential conversion element.
The first voltage conversion circuit has a gain control terminal, and converts first and second reference input voltages Vref1 and Vref2 to first and second differential output voltages.
The second voltage conversion circuit has the gain control terminal, and converts the first reference input voltage Vref1 and a control input voltage Vc to a third differential output voltage.
The gain control voltage generating circuit includes a pair of MOS transistors operating in weak inversion area and a current mirror circuit connected to the MOS transistors. The first and the second differential output voltages are inputted to the MOS transistors respectively, so that drain currents I1 and I2 flow in the MOS transistors. One of the drain voltages of the MOS transistors serves as the gain control voltage of amplification.
The third differential output voltage is inputted to the exponential conversion element.
The exponential conversion element transforms the third differential output voltage to the output current IOUT which varies exponentially.
When a size ratio of the MOS transistors constituting the current mirror circuit is expressed as m:n, the ratio of drain currents I1 and I2 is shown with the following expression.I2/I1=n/m  (1)
Therefore, the output voltage of the gain control voltage generating circuit varies according to this ratio m:n.
The third differential output voltage of the second voltage conversion circuit also varies according to this ratio m:n, because the gain of the second voltage conversion circuit is controlled by the gain control voltage.
Therefore, the logarithm of output current IOUT of the exponential conversion element which inputted the third differential output voltage is shown like a following expression with relation to the control voltage Vc.ln(IOUT)=ln(I0)+{½·ln(n/m)/(Vref2−Vref1)}·(Vc−Vref1)  (2)
Here, I0 is an inherent constant current of the voltage conversion circuits 1A and 1B.
Since inclination of the output current characteristic is determined by size ratio m:n of the MOS transistor of the current mirror circuit, inclination of the exponential conversion characteristic of the output current to control voltage may be influenced by the inclination of the output current characteristic as shown in the expression (2), when the size ratio varies according to manufacturing conditions etc.
As an example, a change of the output characteristic of output current IOUT (logarithm) to the control voltage Vc is shown in FIG. 1, when the size ratio m:n of MOS transistors shifts +10% or −10% from designed ratio.
As shown in the FIG. 1, when the size ratio m:n of the MOS transistors varies inclination of the exponential conversion characteristic of the output current to control voltage may change according to the variation of the size ratio.